Programmable controllers are a common type of industrial computer for operating a variety of manufacturing equipment, such as assembly lines and machine tools, in accordance with a stored control program. The program comprises a series of process control instructions which are read out and executed to examine the condition of selected sensing devices on the controlled equipment, and to energize or deenergize selected operating devices contingent upon the status of one or more of the examined sensing devices.
The state of many sensing and operating devices can be represented by a single bit of data which is manipulated by the control program instructions. Other devices, such as position sensors, provide multiple bits of data representing a condition of the equipment being controlled. For these latter devices, instructions are provided to manipulate bytes and words of data representing the state of the sensing and operating devices. Additional program instructions perform arithmetic operations, timing and counting functions, and complex statistical reporting operations. Such instructions have become quite standardized in the industry and ar directly associated with the elements of a ladder logic diagram which is easily understood by process control engineers. Program panels such as those described in U.S. Pat. Nos. 3,808,612; 3,813,649 and 4,070,702 have been developed to assist the user in developing and editing ladder logic control programs comprised of such programmable controller instructions.
The Allen-Bradley 1771 series programmable controllers are an example of this type of industrial computer. A 1771 programmable controller includes a rack having a number of slots which hold different kinds of functional modules. One slot holds a processor module that stores and executes the user-defined control program. The control program is executed repeatedly thereby performing the same logical operations over and over again for different workpieces being processed by the controlled equipment. Input and output modules, which electrically interface to sensing and operating devices on the controlled equipment, are positioned in other rack slots. The rack has an backplane with conductors to which the modules electrically couple, thereby enabling the exchange of data and control signals. For example, the processor module gathers data from the input modules representing the status of the sensing devices, and issues control commands to the output modules to which the operating devices are connected. The data is gathered and the commands are issued by the processor module over the rack backplane.
Many applications of programmable controllers require relatively high speed processing of input and output data. One example of such processing involves counting inputs for tracking the production of items, such as counting containers passing along a food canning line. In another example, a need may exist to respond immediately to a given input signal by taking the appropriate action, such as quickly clamping a workpiece when its presence is detected. These events may occur more frequently than once every execution cycle through a ladder logic program, occurring while another task is being performed. Therefore, its is desirable to provide a processor module with the capability to monitor selected inputs and perform simple logical operations without requiring processing time of a microprocessor in the module.